1. Used to add sexual innuendo to a conversation.
2. When a male wants to share with his friends that he was with a girl (whether her was or not), he will respond to a phrase that sets up "that's what she said..." as funny or sexual.
3. To imply that what is being talked about was actually a sexual statement and that a girl agreed to said statement in the recent past.
4. Something people say after a phrase is said by someone else to add emphasis.
5. A phrase said by one male to another after a story about her was told to imply that he was with the first male's girl after him.

"thats what she said" is a fun way to work sex into any conversation. heres how:

"she" reffers to a hypothetical girl that one supposedly had sex with, and during or around the act of sex the girl supposedly says something. such as, "chill out, its not like i can get it all in at once". so this joke is useful when your friend says something that sounds like something a girl would say during or around the time of the act of sex. if you do it right you can simultaneously indicate that you are having great sex, and also that your girlfriend is totally impressed with your size, or cant get enough of you, or wants to include other girls, or basically anything you want. so be creative in how you use this phrase. definitely play around with it. of course, that's what she said.

innocentgirl: and leave my mother out of this
phote: thats what she said
innocentgirl: that's what who said?
phote: wow you've never heard "thats what she said" before? check urbandictionary.com
innocentgirl: still not gettin it
phote: ok so for example:
phote: you said "leave my mother out of this." by replying with "thats what she said," i was insinuating that my girlfriend had rejected my invitation to include her mom in one of our sexual encounters.
innocentgirl: oh dear. ok. I get it. wow. I feel so violated...
innocentgirl: lol
innocentgirl: wait
innocentgirl: that's what she said, right?
phote: i think you've got it...
innocentgirl: did she say that, too?
phote: haha yes
innocentgirl: so i guess "she" can say just about anything right?
phote: correct. which is why this phrase is so fun.